Tail Light Issue

[Cooper197]

Hey All

On my '66 long bed, I'm having issues with the rear lights. When I turn on the parking lights or headlights, I'm only getting 6.7 volts to the rear lights. Everything else works fine, all headlights work, the heater fan works, the stereo works, no issues. Should note, I get 6.7 volts with all other electronics on or off.

I rewired the truck over the winter. I had this issue before but assumed shoddy wiring. None of the old wiring is left. It is 100% new and the problem persists.

What is really strange is that after I drive it for a while, they seem to start working just fine. I assumed this was due to a cheap 1-wire alternator. Replaced it with a high quality 3-wire which seems to have made no difference.


Any ideas on how to solve this?

[CousinTone]

Originally Posted by Cooper197

Hey All

On my '66 long bed, I'm having issues with the rear lights. When I turn on the parking lights or headlights, I'm only getting 6.7 volts to the rear lights. Everything else works fine, all headlights work, the heater fan works, the stereo works, no issues. Should note, I get 6.7 volts with all other electronics on or off.

I rewired the truck over the winter. I had this issue before but assumed shoddy wiring. None of the old wiring is left. It is 100% new and the problem persists.

What is really strange is that after I drive it for a while, they seem to start working just fine. I assumed this was due to a cheap 1-wire alternator. Replaced it with a high quality 3-wire which seems to have made no difference.


Any ideas on how to solve this?

How well is the circuit grounded? Is the ground wire on top of paint? Go over each area that should be grounded and make sure you're ground is good, the right size, clean and in good shape. Other than that, I doubt the wire company is F****ng with you and inserted a resistor in the wire. I would look at ground first. Then check other electrical items in the area and insure they are getting the proper voltage. Driving around may giggle the ground just enough to get full voltage. I'm just giving you where I'd start. i would have done this before spending all that money on am alternator! But i'm sure it's going to serve you well.

Go get it! Electrical gremlins suck, big time. CousinTone

[Cooper197]

Originally Posted by CousinTone

How well is the circuit grounded? Is the ground wire on top of paint? Go over each area that should be grounded and make sure you're ground is good, the right size, clean and in good shape. Other than that, I doubt the wire company is F****ng with you and inserted a resistor in the wire. I would look at ground first. Then check other electrical items in the area and insure they are getting the proper voltage. Driving around may giggle the ground just enough to get full voltage. I'm just giving you where I'd start. i would have done this before spending all that money on am alternator! But i'm sure it's going to serve you well.

Go get it! Electrical gremlins suck, big time. CousinTone

I forgot to mention, I did check all the grounds. I redid the grounds from the lights to the frame (all LED) and made sure to wire wheel the ground point on the frame. Ran new 2 gauge ground cables under the hood including from the battery directly to the frame and again made sure to remove the paint.

My girlfriend just told me that when she was following me earlier today the brake lights were much brighter if I had just been on the gas, but when sitting at a stop light they would start to flicker after about a minute.

I have a decent understanding of wiring, but this one really has me puzzled.

[Captainfab]

If you are talking about the brake lights, check your turnsignal switch. The brake light wires go thru the turnsignal switch.

[Cooper197]

Originally Posted by Captainfab

If you are talking about the brake lights, check your turnsignal switch. The brake light wires go thru the turnsignal switch.

Brake lights function most of the time. Occasionally they flicker which goes away when RPM's climb.
It's the tail lights that are the main issue.

[Ken Cook]

I would start at the front of the harness with a volt meter at the headlight switch for the tailights and at the blinker switch for brake lights. Make sure you have 12v coming in and going out. Then at each connector/plug along the way to the back of the truck on both sides of the connection. Looking for low voltage and also loose connections. I have seen a bad connection give 12v but not give enough current.

I once replaced a radio that showed 12v with a meter. The new radio did the same thing so I checked the fuses. The fuses all showed 12v. This was in a camper that used blade type fuses. I found that the blades had corrosion and still give 12v but not enough current to run the radio.

[TheBrewery]

If you have this asshole, you may need to replace or "rebuild" it. I have gone thru 4-5 of them in just a few months. The pins lift up when you plug them in then the contacts can't slide correctly and cause low voltage or worse they slip off their perches and make random contact when you don't want it.

[damnyankee36]

This won't necessarily fix your problem, but I assume the 6.7 volts is with the bulbs installed? Try seeing what the voltage is without the bulbs installed. Try it with one removed, then both.

[shp4man]

Sounds like a connector somewhere. i have no idea where or how many connectors an aftermarket wiring harness has, but if it was doing this with the old harness look for something common to both.

[Cooper197]

I do not appear to have the plug shown above. The 6.7 volts is measured at the license plate light with no bulb in place. Same lead as the tail lights.

The way the wiring is ran, the wire coming out of the fuse box is soldered directly to the lead on the LED tail lights. On the drivers side, there is an extra wire off that solder joint that attaches to the license plate light. I tested power at the fuse and got 14.5 volts, same as I'm getting at the battery and front lights.

I desoldered the single connection and nothing changed. Drove the truck for 20 minutes then tested again. With the truck running I was getting 14.1 volts to the lights (which were working). After it idling for about 3-4 minutes, I was only getting 6.7 volts again.

[damnyankee36]

Do you get 6.7 volts anywhere else? At the battery or alternator output? If it's that low there then the battery is also at that voltage. It shouldn't even crank.

[shp4man]

LED tailights is a clue. Were they there with the old harness? Some of them need a resistor to keep the flashers from blinking too fast. About 6.7 volts is consistent with that theory.

[Cooper197]

Only 6.7 on the rear tail light circuit.

I have an LED flasher hooked up. I had the LED's before and the correct flasher for them. Blinkers work and flash at the correct speed, the only issue is that they will flicker if the truck is at idle.

I just had an idea that actually could be much of my issue...
The headlight switch was a cheap Autozone one. Often the dash lights will kick off and I'll have to reach under the dash and lightly hit the bottom of the switch for them to come back on. I suppose I could be having the exact same thing happening with my tail lights. I'll have to check what my output voltage from that switch is

[damnyankee36]

Good idea checking the switch. LEDs don't draw much to load down a bad switch. Possible. That's why I asked to check the voltage with all the bulbs removed. If it's a bad switch then usually the load causes the switch contacts to break down.

If it's still at 6.7 volts with all bulbs removed then it's possible there is truly a solid 6.7 volts on the circuit or there is something else loading down the circuit.

You're on the right track by testing the voltage at different points in the circuit.

[Captainfab]

Your dash lights and your tail lights are on the same circuit.

[Cooper197]

Originally Posted by Captainfab

Your dash lights and your tail lights are on the same circuit.

Well sounds like that’s going to be my answer then. I’ll double check tomorrow to be sure.

Any recommendations as to someplace to find a quality switch? Seems like I’ve had issues with the last few from a parts store...
Posted via Mobile Device

[Ken Cook]

The female blade connectors in the headlight switch connector could be loose. I would check them before buying another switch. They curl around and slide over the males in the switch. You can squeeze them a LITTLE bit with needle nose pliers or maybe press on them with a screwdriver. I would do this with the switch disconnected.

[X24]

Originally Posted by Cooper197

Well sounds like that’s going to be my answer then. I’ll double check tomorrow to be sure.

Any recommendations as to someplace to find a quality switch? Seems like I’ve had issues with the last few from a parts store...
Posted via Mobile Device

i ran into some brakelight issues a few months ago.. ended up being my turn signal switch (behind the steering wheel). fixed the majority of my issues.
CLASSIC PARTS in Huntington beach, CA... i got a new AC Delco headlight switch... little pricey ($50-$60) but so worth it!